Positioning optical components and waveguides

ABSTRACT

A method of positioning an optical component such as a laser diode (36) in alignment with an optical waveguide (30) comprises forming an elongate V-shaped groove (29) and a depression (31) in a substrate (28). A laser diode (3) is then mounted in the depression (31) and is accurately located therein. An optical fibre (30) is mounted in the groove (29). The relative positions of the depression (31) and the waveguide (30) are such that in use an optical beam may be coupled between the optical component and the waveguide.

FIELD OF THE INVENTION

The invention relates to methods for positioning an optical componentand at least one optical waveguide in alignment with one another.

BACKGROUND AND SUMMARY OF THE INVENTION

Recent developments in the field of optical communication have lead tothe more widespread use of monomode waveguides such as monomode opticalfibres. It is particularly important in this case to devise a method foraccurately positioning an optical component such as a laser chip orphotosensor in alignment with an optical waveguide.

In accordance with one aspect of the present invention we provide amethod of positioning an optical component and at least one opticalwaveguide in alignment with one another, the method comprising

(a) forming at least one elongate waveguide and a locating formation ina common substrate; and,

(b) mounting an optical component in the locating formation,

the relative positions of the locating formation and the or eachwaveguide being such that in use an optical beam may be coupled betweenthe optical component and the or each waveguide.

By using such a locating groove, we have discovered that it is possibleaccurately to align a component with an optical waveguide since thecomponent is located by the groove which is in the same single crystalas the waveguide. For example, a single crystal of silicon is aparticularly suitable substrate for a number of reasons. These includethe fact that silicon is readily available in large sizes to the mostexacting standards of purity and perfection; photolithographic andetching techniques for defining surface and sub-surface geometries arehighly developed; and most importantly large anisotropic etching ratesexist between different crystallographic axes in the crystal.Furthermore, silicon has particularly useful electrical, mechanical andoptical properties which enable the crystal to be used foropto-electronic sub-assemblies such as electro-optical modulators.

The invention also enables an optical component to be mounted in thesame substrate as other optical devices.

The invention is particularly suitable for use with laser chips whichhave a line emission from an edge of the chip or with optical sensingchips. It has been very difficult up to now accurately to align the edgewith an optical waveguide. However, these chips typically have a pair ofangled supporting surfaces enabling the chip to be accurately positionedwithin the locating groove.

The method may further comprise causing optical radiation to be coupledbetween the optical component and the at least one waveguide, adjustingthe position of the at least one or each waveguide, and monitoring theoptical power coupled between the component and that at least onewaveguide to determine the positions corresponding to maximum powercoupling.

The optical component may typically be bonded in the locating groove byfor example soldering.

The at least one optical waveguide may be formed by diffusing a suitablematerial into the substrate to change the refractive index of thesubstrate or by forming a substantially V-shaped groove in the substrateand mounting an optical fibre in the groove. Other methods are alsopossible.

Conveniently, in the former case, the locating groove is formedsubsequently to the formation of an elongate optical waveguide, wherebythe locating groove divides the elongate waveguide into two subsidiarywaveguides. Alternatively, in the latter case the locating groove may beformed subsequently to the formation of a V-shaped fibre receivinggroove, whereby the locating groove divides the V-shaped fibre receivinggroove into two subsidiary V-shaped fibre receiving grooves, opticalfibres being mounted in respective ones of the fibre receiving grooves.

With either of these methods, two optical waveguides are produced whichare accurately aligned and, in the case where the optical componentcomprises a laser chip, may end adjacent opposite facets of the laserchip. This is useful for a number of reasons. Firstly, access to bothfacets helps to improve the spectral response of the laser chip.Secondly, a single laser chip can be used for both transmitting andreceiving in optical communication systems; and thirdly monitoring ofthe laser chip can be carried out.

In accordance with a second aspect of the present invention, an opticalassembly comprises an optical component and at least one opticalwaveguide which have been positioned in alignment with one another by amethod in accordance with the first aspect of the invention.

In another arrangement, two optical waveguides are provided, adjacentends of the waveguides being laterally offset, and wherein the locatinggroove intersects the adjacent ends of the waveguides such that theoptical component positioned in the locating groove may be opticallycoupled with both waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

Some examples of methods and assemblies in accordance with the inventionwill now be described with reference to the accompanying drawings, inwhich:

FIG. 1 is an exploded, perspective view of a first example;

FIG. 2 is a side elevation of the example shown in FIG. 1;

FIGS. 3 and 4 are side elevations similar to FIG. 2 but of modificationsof the first example;

FIG. 5 is a perspective view of a second example; and,

FIG. 6 is a view taken in the direction A in FIG. 5.

DETAILED DESCRIPTION

The example shown in FIG. 1 comprises a single crystal silicon chip orsubstrate 28 having an elongate V-shaped groove 29 for receiving anoptical fibre 30. A depression 31 having a triangular prismatic form isetched into the substrate through an intermediate portion of the groove29 using an anistropic etching technique, which produces a self limitedgroove. The depth of the depression 31 is chosen in a conventionalmanner by defining the surface area of the substrate 28 which is etched.The depth is chosen such that the included angle between opposite faces32, 33 of the depression 31 is accurately known. The detector chip 36would need to be fabricated to produce a compatible included anglebetween its side walls 34, 35 and this could be achieved by using adiamond saw set at the required angle to divide up a silicon slice intoseparate chips. The depth is also chosen such that simply by insertingthe chip 36 into the depression 31, the optical fibre 30 will be alignedwith the centre of the 30 to 50 micron light sensitive area of the chip36. As previously mentioned, the alignment between the chip 36 and theoptical fibre 30 is set by the relative depths of the V-shaped groove 29and the depression 31 which are defined by the surface window size usedfor the self-limiting edge.

Furthermore accurate alignment between the groove 29 and depression 31is achieved by etching them together using different window sizes. Thefact that the grooves are self limited means that the etching processdoes not have to be timed, as it is not possible for over etching tooccur. This greatly simplifies the process in comparison with otherknown etching techniques which require accurate timing of the etching inorder to define the required shape and size of the groove.

The chip 36 and the fibre 30 could be attached to the silicon substrate28 by suitable metallisation and bonding. The chip 36 is typically 500μm square and 200 μm thick.

FIG. 2 illustrates the use of the angled end surface 37 of thedepression 31 as the combined bonding and lower contacting surface. Anyoptical penalty caused by the angled detector surface will be minimisedby the use of an anti-reflection coating on the chip 36 and the lensended fibre 30.

If required, a mirror image substrate 38 defining a clamp could bemounted on the substrate 28 as shown in FIG. 3. The grooves 29 in eachsubstrate 28, 38 could be used to provide a route for the secondelectrical connection to an outside circuit but other configurationsincluding metal tracks on the surface of one or both of the substrates28, 38 are possible.

An extension of the principle shown in FIG. 3 would allow a balancedphotodiode configuration to be considered with the fibres in-line asshown in FIG. 4, using a parallel arrangement of grooves and fibres. InFIG. 4 a second detector 36' is bonded to the face of the depression 31opposite to the face 37 and each detector 36, 36' is received incomplementary depressions 39, 40 in the upper substrate 38. In additiona second optical fibre 30' extends between the substrate 28, 38 to thedetector 36'.

For optical receivers where the first stage amplifier is a bipolarsilicon transistor or FET the associated circuits could be diffused intothe silicon chip 28 adjacent to the photodiode 36.

In contrast to the light sensitive area of a photodiode, the lightemission from a laser diode is from a region situated on two oppositesides of the chip and is situated about 1.5 microns from one of thefaces. One of the main requirements in the mounting of a laser diode isthat the centre line of the light emitting region is aligned with theaxis of the adjacent optical fibre. FIGS. 5 and 6 illustrate a furtherassembly for achieving this mounting configuration. In this example, twoV-shaped grooves 41, 42 are etched in a silicon substrate 43, the twogrooves being parallel but offset from one another. A laser diode chip46 is bonded to a sloping side 44 of the groove 42 while an opticalfibre 45 is positioned in the groove 41. As before, the depths of thegrooves 41, 42 are defined by a horizontal masking process andadditionally the fibre diameter and concentricity is accurately defined.Thus, the alignment accuracy is dictated by the width of the chip 46.The lens ended optical fibre 45 is tapered to allow an efficient fibreto laser distance to be obtained.

I claim:
 1. A method of positioning an optical component and at leastone optical waveguide in alignment with one another, said opticalcomponent having at least two supporting surfaces the method comprisingthe steps of:(a) forming at least one elongate waveguide and a locatinggroove in a common single crystal substrate, the step of forming thelocating groove including anisotropically etching the substrate to forma self-limited V-shaped groove, the included angle of the locatinggroove being substantially equal to the angle between the supportingsurfaces of the optical component; and (b) mounting the opticalcomponent in the locating groove; the relative positions of the locatinggroove and the or each waveguide being such that in use an optical beammay be coupled between the optical component and the or each waveguide.2. A method according to claim 1, wherein the at least one waveguide isformed by diffusing a suitable material into the substrate.
 3. A methodaccording to claim 2, wherein the locating groove is formed subsequentlyto the formation of an elongate optical waveguide, whereby the locatingformation divides the elongate waveguide into two subsidiary waveguides.4. A method according to claim 1, wherein the at least one waveguide isformed by forming a substantially V-shaped fibre receiving groove in thesubstrate and mounting an optical fibre in the fibre receiving groove.5. A method according to claim 4 wherein the locating groove is formedsubsequently to the formation of the V-shaped fibre receiving groovewhereby the locating groove divides the V-shaped fibre receiving grooveinto two subsidiary V-shaped fibre receiving grooves, optical fibresbeing mounted in respective ones of the fibre receiving grooves.
 6. Anoptical assembly comprising an optical component having at least twosupporting surfaces and at least one optical waveguide which have beenpositioned in alignment with one another by the method of:(a) forming atleast one elongate waveguide and a locating groove in a common singlecrystal substrate, the step of forming the locating groove includinganisotropically etching the substrate to form a self-limited V-shapedgroove, the included angle of the locating groove being substantiallyequal to the angle between the supporting surfaces of the opticalcomponent; and (b) mounting the optical component in the locatinggroove; the relative positions of the locating groove and the or eachwaveguide being such that in use an optical beam may be coupled betweenthe optical component and the or each waveguide.
 7. An assemblyaccording to claim 6, wherein the optical component comprises a laserchip.
 8. An assembly according to claim 6 wherein the optical componentis bonded to a surface of the locating groove.
 9. An assembly accordingto claim 6, wherein two optical waveguides are provided, adjacent endsof the waveguides being laterally offset, and wherein the locatinggroove intersects the adjacent end of the waveguides such that theoptical component positioned in the groove may be optically coupled withboth waveguides.
 10. An optical assembly according to claim 6 whereinthe locating groove provides for a transverse offset with respect to theoptical waveguide axis.
 11. An optical assembly according to claim 6wherein the locating groove provides for an offset with respect to thewaveguide axis.
 12. An assembly according to claim 6 wherein the opticalcomponent comprises an optical sensing chip.